Which carbs should i run?

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apsracing

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Building a 78 xs650 brat. Wondering which carbs are better. Motor is stock, but has pandemonium pipes. I have the stock carb rack from the 78. Also have another set from a an 81 xs. Both need to be cleaned and tuned. Just wondering which i should use. Any info would be great .. Thanks..
 
I'd go with the 78 carbs in my opinion .They're 38 mm carbs. The 81 carbs would be 34's.
I'm sure there will be others with differing ideas.
 
I like the 38's. But i have more experience with them than i do the 34's. There is alot of info on here about carbs, 5twins has posted some things that really helped me out.:thumbsup:
 
From what I have found by running both a set of 76-77 BS38 and an 82 set of BS34 carbs. The BS38's run a bit stronger at higher rpms but the BS34's have smoother transitions from idle to mid range. This gives slightly better throttle responce under the conditions most ride at.
Just be aware that the BS38 and BS34 use different carb holders. If you have carb holders for both sets, I would clean and tune both sets. See which you like best.
Leo
 
Thanks for all the tips. looks like i better start rebuilding both.
Anyone know of any good jet kits or tuning settings for the rebuilds.
I also need floats for the 38's.
 
apsracing said:
Thanks for all the tips. looks like i better start rebuilding both.
Anyone know of any good jet kits or tuning settings for the rebuilds.
I also need floats for the 38's.
Click to expand...
....................hi guys i favour the 38,s i had mine on my bike for 10 years with mikesxs jets in them,,,, they are the kit you buy and you get every thing ,,and i,ve never had dramas,,,, plus you can adjust the 38,s with a hammer ... or pretty much anyway regards oldbiker
 
You have what I consider the two best stock carb sets put on the 650. Either should serve you well. Many consider the '76-'77 38s the best but I like the '78-'79 set a little better. They are smoother through the low end and midrange like the 34s. I think this is due to both sets having a spring loaded needle which makes it meter more precisely. The earlier 38s retain their needle in the slide with a large plastic disc. There is play between it and the needle clip, about a half a MM. That means the needle can (and does) move up and down the equivalent of a half clip position while operating. Obviously, not a very precise way to meter your fuel flow.

There is no such thing as a "jet kit" for these bikes. The carbs changed too much through the production run. There could not be a "one size fits all" kit, there would have to be 3 or 4 different ones - and that's just not economically feasible for an out-of-production "niche" bike like this. There are aftermarket rebuild kits but the quality of some of the included parts is questionable. The cheapo floats sold by Mike's are reported to be junk, springing leaks in a short time. Although expensive, genuine replacements are best.
 
On most rebuild kits the brass parts are generic and probably won't work very well. The gaskets will work ok.
Often you can tear apart the carbs and reuse the parts you have. The float bowl gasket is very tough and you can pull the bowl many times as long as you don't use a gasket sealer. Sealers glue the gasket to both surfaces and tears the gasket. If you feel they need something on them a thin coat of grease or anti-sieze compound seals well and won't tear gaskets.
Your brass parts clean up well.
As far as jets, you can get them at most any dealer or independent shop. Usually genuine Mikuni parts. And at about the same price as our favorite online parts supplier.
Often independent shops have an assortment of used jets from rejetting dirt bike, atv's and such. Might even get them a bit cheaper.
www.amckayltd.com/carbguide.pdf is the bible for your carbs.
Depending on your mods some rejetting will be required. The carb guide has a section on tuning.
Leo
 
If you are serious about a good running bike, I replace the throttle shaft seals on both 34s and 38s, What are the odds of a 30+ year old pliable working rubber seal?
 
I've always favored a smaller throat diameter on mid-displacement bikes. Many tuning manuals, and experience cited here backs it up to an extent, favor the smaller opening for low end and midrange, where most of us live most of the time, VS the outright lower restriction of the larger diameter.

I worked on an early Honda Thumper XL600R big single (kickback king!) that had 2 carbs. Had a 28MM carb for low to mid-range, and then the other 28MM opened up for the top end. It was called "compound" carbs, I think.
As suggested, I'd go after both. If you can get both sets worked through for little scratch, you can take your pick and sell your spares if you want, or hang on to them. If you have not started yet, it's possible to find things inside that make your decision for you.
 
I would use the set that are easiest to take apart, half the time someone has taken the edges off the jets and they aren't going any where,
 
I have to agree on the 34s too. I have had both and the 34's arent near as bad as what people say. Especially on a stock or close to bike. I run uni pods and mikes brit mufflers and all i did was up the mains one and leave the rest alone and it seems to run and pull great. Also have a pamco. Plus 34's are in abundance and you can even take the 34's from ninja 500's and adapt them to your use. So tons to pick from.
 
I always thought that the general consensus was that the linked BS38s (1976-79) were hands-down the best, and I'm a little surprised that the gurus who have responded have not supported this. Anyways, I have re-built plenty of both sets, and here is why I like the BS38s best:

The BS38's have an adjustable slide needle, BS34s do not. This seems like a serious oversight to me. A lot of us believe that the BS34s were a response to EPA requirements, and were never intended to be tampered with, and that Mikuni built them accordingly.

It is very easy to adjust the air mix screw on a running BS38. On a BS34 you first have to drill out the air mix screw cap, and even then, it is very awkward to adjust the screw, and you burn your hand on the engine while trying to do so. See the EPA comment above.

The main jet of a BS38 can be changed without removing the float bowl. The pilot jet of a BS38 is in the float bowl, so it is also very easy to change. Replacing either jet on a mounted BS34 is more awkward.

BS38s look better. The stamped metal diaphragm caps on BS34s are cheesy-looking and the chrome will eventually rust and pit. The caps on BS38s are cast aluminum, look great, and can always be polished to a mirror finish. The rest of the BS38 body looks better too in my opinion, better curves and lines. Plus, a lot of BS34s are painted black, which eventually chips and wears and fades and looks ratty.

The one significant advantage of the BS34s is that the choke (actually an enrichener) is adjustable. The BS38 choke is either all on or all off.
Also, the BS34 float valve has a screen which the BS38 doesn't, but you should be running an in-line fuel filter, so this shouldn't matter.
Also, on the 1978-79 BS38s you need a special tool to get to the slide needle.
 
"Also, on the 1978-79 BS38s you need a special tool to get to the slide needle."
Yeah, Tell me about it! That clip was a pain to get to.:laugh:
 
First. thanks for all of the great info. My plan is to start rebuilding both and go fromthere. I figure if I have two good sets and can't decide. Maybe I will just have to build a hardtail for the other carbs......
 

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